In the digital coherent optical transmission system, an optical signal which propagates through an optical fiber transmission path is coherently received, the received signal is converted into an electric signal, and then, waveform equalization processing such as wavelength dispersion compensation or the like is performed to the electric signal by digital signal processing. By performing this process, the waveform distortion of a signal light which occurs when the signal light is transmitted at an ultrafast speed through an optical fiber can be compensated with a high degree of accuracy. In the digital coherent optical transmission system, an expensive optical component for wavelength dispersion compensation such as a DCF (Dispersion Compensating Fiber), a TDC (Tunable Dispersion Compensator), or the like is unnecessary.
It has been studied to apply the digital coherent optical transmission system to, for example, a metro access system with a maximum transmission distance of about 80 km or a 100 Gbps (gigabit per second) Ethernet (registered trademark) signal transmission system used for an intra-data center network or an inter-data center network. In particular, it is required to realize such short and middle distance transmission system at a low cost. For this reason, a (single-core bidirectional transmission) configuration in which an optical signal is bi-directionally transmitted and received through one optical fiber, a configuration in which one light source is shared for a light source for transmission and a local light source for reception, and the like are used. When the configuration in which one light source is shared for the light source for transmission and the local light source for reception is used, the wavelength of the transmission signal light is equal to the wavelength of the reception signal light (same-wavelength bidirectional transmission). Further, even when the light source for transmission and the local light source for reception are independently provided, the wavelength of the light source for transmission may be made equal to the wavelength of the local light source for reception to simplify wavelength management.
FIG. 5 is a block diagram showing an example of a configuration of a common optical transmission system 900. The optical transmission system 900 includes optical transceivers 910 and 911 and realizes same-wavelength single-core bidirectional transmission. The optical transceivers 910 and 911 are connected to each other through a single-core optical fiber transmission path 20. A transmission signal 100 is inputted to the optical transceiver 910. A signal processing circuit 810 encodes the transmission signal 100, converts it into a digital signal, performs digital signal processing such as filtering and the like, and performs DA (Digital to Analog) conversion to generate a modulation signal 120. The modulation signal 120 is inputted to an optical modulator 130. The optical modulator 130 modulates a transmission light 150 outputted by a light source 140 by the modulation signal 120 and outputs the modulated signal as a signal light 160. The signal light 160 is outputted to the optical fiber transmission path 20 through an optical coupler 170 and transmitted to the optical transceiver 911.
The optical transceiver 910 receives a signal light 161 from the optical transceiver 911. The signal light 161 is inputted to an optical demodulator 200 through the optical coupler 170. The optical demodulator 200 mixes a local light 210 outputted from the light source 140 with the signal light 161 and outputs a reception signal 220 that is an electric signal. The reception signal 220 is inputted to a signal processing circuit 830. The signal processing circuit 830 performs AD (Analog to Digital) conversion, digital signal processing such as wavelength dispersion compensation, polarization separation, and the like, a decoding process, forward error correction (FEC), and the like to the reception signal 220 and outputs the resultant signal as a transmission signal 101.
The optical transceiver 911 has a configuration and a function that are similar to those of the optical transceiver 910. Namely, the transmission signal 101 is inputted to an optical transceiver 911. A signal processing circuit 811 encodes the transmission signal 101, converts it into a digital signal, performs digital signal processing such as filtering and the like, and performs DA conversion to generate a modulation signal 121. The modulation signal 121 is inputted to an optical modulator 131. The optical modulator 131 modulates the transmission light 151 outputted by the light source 141 by the modulation signal 121 and outputs the modulated optical signal as the signal light 161. The signal light 161 is outputted to the optical fiber transmission path 20 through an optical coupler 171 and transmitted to the optical transceiver 910.
The optical transceiver 911 receives the signal light 160 transmitted from the optical transceiver 910. The signal light 160 is inputted to an optical demodulator 201 through the optical coupler 171. The optical demodulator 201 mixes a local light 211 outputted by the light source 141 with the signal light 160 and outputs a reception signal 221 that is an electric signal. The reception signal 221 is inputted to a signal processing circuit 831. The signal processing circuit 831 performs AD conversion, digital signal processing such as wavelength dispersion compensation, polarization separation, and the like, a decoding process, forward error correction (FEC), and the like to the reception signal 221 and outputs the resultant signal as the transmission signal 100.
In relation to the present invention, patent literature 1 (Japanese Patent Application Laid-Open No. 2001-160778) discloses a technology for suppressing deterioration in reception light sensitivity due to interference light in an optical transmission and reception system in which the wavelength of the reception light is different from the wavelength of the transmission light. Patent literature 2 (Japanese Patent Application Laid-Open No. 2004-242224) discloses a technology for reducing a reflected wave component that appears in the reception signal when the signal transmitted by the transmitter is reflected at a reflection point of an optical fiber transmission path and returns to the receiver on the transmission side.